A solid oxide fuel cell comprises an anode electrode, a cathode electrode and an electrolyte between the anode electrode and the cathode electrode. A solid oxide fuel cell stack comprises a plurality of solid oxides connected in electrical series or in electrical parallel arrangements. In a solid oxide fuel cell a gaseous fuel, for example hydrogen, is supplied to the anode electrode and a gaseous oxidant, for example oxygen or air, is supplied to the cathode electrode.
The electrolyte of a solid oxide fuel cell is required to form a physical barrier between the gaseous fuel supplied to the anode electrode and the gaseous oxidant supplied to the cathode electrode. The electrolyte comprises a dense non-porous layer. Any gaseous leak paths through the electrolyte between the anode electrode and the cathode electrode will allow the gaseous fuel and oxidant to come into contact. This leakage produces a reduction in the solid oxide fuel cell performance in terms of fuel utilisation and may detrimental to mechanical integrity and durability of the solid oxide fuel cell. It may even result in combustion of the gaseous fuel in the gaseous oxidant and possibly severe damage to the solid oxide fuel cell.
The electrolyte, in practice, has defects, or flaws, in its microstructure. These defects, or flaws, may be caused by contamination and/or variability in the manufacturing process. It may be possible to minimise these defects, or flaws, by implementing greater manufacturing process control, however, complete elimination of the defects, or flaws, is likely to result in a tightly controlled manufacturing process and hence an expensive manufacturing process.
One possible solution to the problem was to provide a further dense non-porous layer on the existing dense non-porous layer of the electrolyte. However, in practice the original defects in the dense non-porous layer propagate into the further dense non-porous layer resulting in no benefit.